1) Field of the Invention
The present invention relates to a technology for generating a pixel clock in a laser printer, a digital copying machine, and other image forming apparatuses, and for generating a pulse modulation signal as a laser drive signal.
2) Description of the Related Art
FIG. 16 is a general configuration diagram of an image forming apparatus such as a laser printer and a digital copying machine. As shown in FIG. 16, a semiconductor laser unit 1001 emits a laser beam, a rotary polygon mirror 1002 scans the laser beam, and laser beam passes a scan lens 1003 to form a light spot on a photosensitive element 1004 as a scanned medium, thereby exposing the photosensitive element 1004 to form an electrostatic latent image. Here, based on a signal output for each line from a photodetector 1005, a phase-synchronized image clock (a pixel clock) is generated for each one line, and is supplied to an image processing unit 1006 and a laser drive circuit 1007 respectively. The semiconductor laser unit 1001 controls the time of emitting a semiconductor laser beam in this way, following image data generated by the image processing unit 1006 and the image clock of which phase is set for each one line by a phase synchronizing circuit 1009. With this arrangement, the semiconductor laser unit 1001 controls the electrostatic latent image on the scanned medium 1004.
In this scan optical system, a variance in the distance from a polarizer such as a polygon scanner to a rotation axis of a polarization reflection plane generates unevenness in the scan speed of a light spot (a scan beam) that scans a scanned surface. This scan speed unevenness generates fluctuation in the image, and degrades image quality. When high-definition image is required, it is necessary to correct the scan unevenness.
In a multi-beam optical system, when oscillation wavelengths of lights emitted from various light sources are different, an exposure positional deviation occurs in the optical system in which chromatic aberration of the scan lens is not corrected. When a spot light from each light emission source scans the scanned medium, scan widths of lights become different between those from different light emission sources. This difference in scan widths degrades the image quality, therefore the scan widths must be corrected.
Conventionally, to correct scan unevenness and the like, a light spot position along a scan line is controlled by changing a frequency of a pixel clock. Such a technology is disclosed in, for example, Japanese Patent Application Laid-Open Nos. H11-167081 and Japanese Patent Application Laid-Open No. 2001-228415. In another method, a scan variance is corrected by controlling a phase of a pixel clock. Such a technology is disclosed in, for example, Japanese Patent Application Laid-Open No. 2003-98465. There is also a method of achieving fine and high gradation of an image even when an operation speed is fast, by optionally generating a pulse modulation signal having a desired pattern as a laser drive signal. Such a technology is disclosed in, for example, Japanese Patent Application Laid-Open No. 2003-103831.
However, according to the conventional system (the frequency modulation system) of changing a frequency of a pixel clock, configuration of a pixel clock controller is generally complex. This complexity increases along a gradual fining of a frequency modulation width, therefore a fine control can not be carried out. According to the conventional system of controlling a phase of a pixel clock or the conventional system of generating a pulse modulation signal in a desired pattern, the control is carried out individually, therefore the length of one pixel can not be controlled in high precision.